The present invention relates to the fabrication of semiconductor devices. More particularly, the present invention relates to improved techniques for forming contact holes through to a silicon layer of a substrate in a plasma processing chamber.
In the fabrication of semiconductor devices (e.g., integrated circuits or flat panel displays), contact holes, such as trenches, vias, and the like, may sometimes be formed through an oxide layer to a silicon layer of a substrate (e.g., a silicon wafer or a glass panel). It is known that such contact holes may be etched in a plasma processing chamber wherein a plasma that is capable of etching the oxide material through openings in a photoresist mask is utilized.
To facilitate discussion, FIG. 1 depicts a simplified layer stack 100, including a silicon layer 102, oxide (i.e., SiO.sub.2 or a material that contains SiO.sub.2) layer 104 and photoresist mask 106. To simplify discussion, only some exemplary layers are shown. As is well known, however, other layers (including, for example, adhesion layer, seed layer, anti-reflective coating layer, or another layer) may also be disposed above, below, or in between the shown layers. Silicon layer 102 represents, in this example, a monocrystal silicon layer that may be disposed above a substrate or may even represent the monocrystal silicon substrate itself. In photoresist mask 106, an exemplary opening 108 is shown through which the etching plasma may enter to remove material from oxide layer 104 to form the desired contact hole.
In FIG. 2, a contact hole 202 is shown formed through oxide layer 104 down to the interface between oxide layer 104 and silicon layer 102. Typically, contact hole 202 is etched out of a plasma that is formed using a fluorocarbon-based etchant source gas. By way of example, suitable etchant source gases employed to etch the contact hole through oxide layer 104 may include CHF.sub.3 or CHF.sub.3 /C.sub.4 F.sub.8. When energized, the fluorocarbon etchant source gas forms carbon species and fluorine species to etch the areas of oxide layer 104 that are not protected by photoresist mask 106. By timing the etch or providing an endpoint, the etch may be stopped at about the interface between oxide layer 104 and silicon layer 102.
It has been found, however, that the etching of contact hole 202 leaves a damaged region at the bottom of contact hole 202. By way of example, the bottom of contact hole 202 may have a layer of amorphous silicon having adsorbed C, H, or F. In FIG. 2, this damaged region is shown as damaged region 204 at the bottom of contact hole 202.
The presence of damaged region 204 unfortunately increases the contact resistance between the conductive material that is subsequently deposited into contact hole 202 and the contact region within silicon layer 102 (e.g., a doped well). The increased contact resistance reduces the electrical performance of the device formed on the substrate by, for example, reducing its operating speed or increasing its power consumption. If damaged region 204 is sufficiently thick, the contact resistance may be great enough to render the resultant device defective.
In the prior art, the contact resistance due to damaged region 204 may be reduced by removing some of damaged region 204 in a separate etch process known as a soft etch. The soft etch process, which is typically a separate etch step from the main contact etch that is employed to etch through oxide layer 104, is typically performed using a gas mixture containing fluorocarbons.
Following the soft etch, another separate stripping step is employed to strip photoresist mask 106, as well as adsorbed etch byproducts that are formed on interior surfaces of the plasma processing chamber during the main contact etch. The stripping step typically employs O.sub.2 as the main etchant source gas. Alternatively, some prior art processes perform a separate stripping operation prior to performing the soft etch. In high density plasma processing chambers (i.e., those producing plasma with ion density greater than about 10.sup.13 ions/cm.sup.3), the stripping and main contact etch operations are typically performed in the same plasma processing chamber to allow the chamber to be cleaned of the main contact etch byproducts while the photoresist mask is stripped.
The requirement of three separate processing steps to etch a contact hole through the oxide layer (i.e., a first main contact etch, a soft etch, and a stripping operation) has been found to be disadvantageous as these three separate processes tend to be time consuming. With the prior art technique, the throughput of substrates through the plasma processing chamber is relatively low, which disadvantageously increases the cost ownership of the etch tool. If these three separate processing steps are performed in different processing systems, additional costly equipments may be required, further driving up the cost of producing the semiconductor-based products.
In view of the foregoing, there are desired improved techniques for forming contact holes through to a silicon layer of a substrate in a plasma processing chamber.